The removal of halogens, and particularly chemically-combined halogens, such as organochlorides, from refinery streams is highly desirable in order to prevent significant equipment corrosion and impurity problems in downstream processing. This is particularly useful in a catalytic reforming unit which contains a bi-functional catalyst with acid function delivered from chlorided alumina. The chloride level of this catalyst must be maintained by constant addition of a chloriding agent, and chlorides continuously leach off the catalyst into the product streams.
Reforming is a process generally known to the petroleum industry as a process for the treatment of naphtha fractions of petroleum distillates to improve their octane rating by producing aromatic components from components present in the naphtha feedstock. Reforming is a complex process and involves a number of competing processes or reaction sequences. These include dehydrogenation of cyclohexanes to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, dehydrocyclization of an acyclic hydrocarbon to aromatics, and hydrocracking of paraffins to light products boiling outside the gasoline range. In addition, the dealkylation of alkylbenzenes and the isomerization of paraffins occur in reforming processes. Some of the reactions occurring during reforming are not desirable owing to their deleterious effect on the yield of commercially valuable products or upon the octane of the products. For example, hydrocracking reactions produce light paraffin gases, e.g., C1-C4, and reduce the yield of products boiling in the gasoline range.
The interest in catalytic reforming processes is fueled by a desire to improve the production (yield) of the gasoline fraction while concurrently increasing its octane.
Naphtha reforming may also be utilized for the production of benzene, toluene, ethylbenzene and xylene aromatics. A valuable by-product of naphtha reforming is hydrogen, which may be utilized for hydrotreating and upgrading of other hydrocarbon fractions. Generally, the molecular rearrangement of molecular components of a feed, which occurs during reforming, results in only slight, if any, changes in the boiling point of the reformate (the product of reforming), compared to that of the feed. Accordingly, reforming differs from both cracking and alkylation, both refinery processes, each of which does result in changes of boiling range of the product compared to the feed. That is, in cracking, large molecules are cracked into smaller ones; whereas, in alkylation small molecules are rebuilt into larger molecules.
The most important uses of the reforming process are briefly mentioned: the primary use of catalytic reforming may be concisely stated to be an octane upgrader and a route to premium gasoline.
Catalytic reforming is the only refining process that is capable of economically making a gasoline component having high clear research octane ratings. The charge to the reformer (straight-run, thermal, or hydrocracker naphtha) is usually available in large quantities and is of such low quality that most of it would be unsaleable without reforming.
Hydrogen, although often considered a by-product, is still a valuable output from the reformer. Normally, it is produced in amounts ranging from 300 to 1200 SCF/Bbl, depending on the type of feed stock and reformer operating conditions. Reformer hydrogen is used to remove unwanted contaminants from reformer feed stocks, for hydrodesulfurization of distillates, hydrocracking of heavy fractions, hydrotreating of lubes and various chemical operations.
All of the reforming catalysts in general use today contain platinum supported on an alumina or an alumina-silica base. In many cases, rhenium is combined with platinum to form a more stable catalyst which permits operation at lower pressures. Platinum is thought to serve as a catalytic site for hydrogenation and dehydrogenation reactions and halogenated alumina provides an acid site for isomerization, cyclization, and hydrocracking reactions. Halide, particularly chloride, is known to be a catalyst promoter when added to a reforming catalyst in catalyst-promoter quantities. While most feeds contain small amounts of halide, it is usually not enough to adequately maintain catalyst activity. Consequently, platinum-containing alumina-based reforming catalysts are manufactured having a predetermined amount of halide, particularly chloride, on catalyst, sometimes up to about 3 wt. %, depending on the active metals content of the catalyst. As the catalyst ages, chloride loss becomes appreciable and, inter alia, contributes to loss of catalyst activity. Various approaches have been taken to address the need for maintaining desirable levels of halide on catalyst, as well as preventing its loss when onstream. Typically, dry halide, in the form of a halogen, halide acid, or an alkyl halide, is injected into one or more series reactors to maintain the halide concentration within the desired range.
Hydrogen is an important by-product from the reforming reaction. Part of the hydrogen stream is recycled and typically the chloride level of this stream is monitored as an indicator of the chloride on the catalyst. Since the reformer generates an excess of hydrogen, a net-hydrogen stream is produced from the reformer. Thus, two chloride-containing streams are produced from the reformer, e.g. net-hydrogen and the reformed naphtha. Net-hydrogen is utilized in many parts of the refinery, as above described and, thus, the net-hydrogen stream is treated to remove the chloride contaminants. Removal of chlorides from the hydrogen stream represents the majority of applications for chloride removal.
The chlorides are generally in the inorganic form, HCl. However, some refiners have reported organic chlorides as well. Since trace levels of C2 to C4 olefins are present in the hydrogen stream, these organic chlorides are presumed to be C2 to C4 chlorinated hydrocarbons such as ethyl chloride, propyl chloride, butyl chloride, vinyl chloride, etc. The levels of these chloride species can vary greatly, but are typically 1-10 ppmv. Inorganic chlorides are very corrosive and can cause significant operating issues in any downstream equipment. Organic chlorides, while not particularly corrosive, decompose at relatively low temperatures to HCl and the corresponding hydrocarbon. Thus, these organic chlorides are suspect when corrosion is observed in streams where the inorganic halides have been removed.
Refiners typically abate the chloride compounds by passing process streams through a fixed bed of adsorbent with specificity for the contaminants of interest. Inorganic chlorides are effectively removed to levels well below 1 ppm using aluminas promoted with alkali (U.S. Pat. No. 5,316,998). In contrast, organic chlorides are more difficult to remove and there is limited evidence of an effective adsorbent in the literature. U.S. Pat. No. 3,862,900 teaches that molecular sieves, especially type 13X, have an affinity for organic chlorides and can be utilized in an adsorption system to effectively separate organic chlorides from a process stream. Reformate process streams are not mentioned as the adsorbent is described as particularly useful to adsorb chemically combined chlorine from hydrocarbon streams produced by the catalytic alkylation of an olefin with an isoparaffin in the presence of a metal chloride catalyst. The Si/Al ratio of said 13X adsorbent is 106:86, or 1.23. In addition, sales literature from companies such as CLS Industries and UOP claim their molecular sieve products are useful in removing organic chlorides from various refinery process streams. Since no mention of Si/Al ratio appears in these references, it must be assumed that these citings refer to a conventional zeolite 13X with a Si/Al mole ratio of 1.25 as defined in “Zeolite Molecular Sieves” by D. W. Breck, R. E. Krieger Publishing Co., 1984, page 316 and pages 278-284 in “Handbook of Molecular Sieves” by Rosemarie Szostak 1992 Van Nostrand Reinhold.